Glossary: Excavation, Septic, Drainage & Material Terms (NH)

Most of these terms come up in the same conversations. A new gravel driveway has a subgrade, a base, a crown, and culverts. A septic install has a perc test, a leach field, a D-box, and a riser. A curtain drain daylights into a swale. None of it's complicated once you know what each piece is doing. This is the cheat sheet.

Excavation & site work

Frost line

a.k.a. frost depth

The maximum depth ground freezes in winter. In central New Hampshire and the Lakes Region, the frost line is about four feet. Foundations, water lines, septic effluent runs, and any buried utility have to clear it. Anything shallower than four feet can heave when the ground freezes and crack what's on top of it. The reason a foundation footing in NH sits on undisturbed soil at four feet of depth (instead of two) is the frost line.

Subgrade

The native soil under whatever you're building on. Subgrade matters because the whole structure rides on it. Compacted, well-draining subgrade holds. Saturated, clay-heavy, or root-disturbed subgrade settles, and whatever's built on top of it eventually does too. On a gravel driveway, the difference between a drive that lasts twenty years and one that fails in three is usually in the subgrade prep, not the visible top course.

Backfill

The material used to fill in around a foundation, a buried utility line, or a septic component after the work is in. Good backfill goes in graded and lift-compacted (in shallow layers, each compacted before the next), not dumped in one drop. The reason matters: dumped fill settles unevenly over the next few years, which pulls on the foundation, breaks the utility line, or shifts the septic riser. Compacted lifts hold.

Glacial till

The dense, mixed-grain soil left across most of New Hampshire when the last glaciation ended. It's a chaotic mix of clay, sand, gravel, and stone, including the occasional house-sized boulder. Glacial till digs slower than sandy soil and can hide ledge or buried boulders that the surface doesn't predict. Most Lakes Region excavation jobs are in some form of glacial till.

Ledge

Bedrock that comes up close enough to the surface to interfere with excavation. NH has a lot of ledge. A foundation footprint that needs to go down four feet for frost protection but hits ledge at two feet has to either be redesigned (slab on grade, raised foundation), or the ledge has to come out (hammer attachment, ripper, sometimes blasting on bigger jobs). Either way the cost moves. A good site visit flags ledge risk before the dig starts.

Septic systems

Perc test

a.k.a. percolation test

The soil test that tells you how fast water moves through the ground. A licensed septic designer or NH-permitted soil scientist digs test pits, runs the test, and uses the result to size and place the leach field. A fast perc means a smaller leach field is fine. A slow perc means a bigger one, or in tough soils, an engineered system. The perc result determines what's possible to build on a lot, which is why a perc test is one of the first things to check on raw land.

Leach field

a.k.a. leaching field, drain field, disposal field

The buried network of perforated pipe and stone that lets septic effluent disperse into the soil for final treatment. The leach field comes after the tank and the D-box; it's where the actual soil treatment happens. A failed leach field is the most common reason a septic system needs replacement. Sized by the designer based on the perc test, the household design flow, and the setbacks required by NH state rule under RSA 485-A.

D-box

a.k.a. distribution box

The small concrete box that sits between the septic tank and the leach field, splitting effluent evenly across the leach field laterals. A D-box that's tipped or that has a clog at one outlet sends too much flow to one part of the leach field, which fails it prematurely. Inspecting and re-leveling the D-box is a routine part of septic maintenance and a common diagnostic step when a system starts showing trouble.

Septic riser

The buried vertical pipe that brings the septic tank's access port up to grade so it can be opened without digging. New installs typically include risers on both the inlet and the outlet. Older systems often don't have them, which means every pump-out requires shovel work to find and uncover the lid. Adding risers to an existing tank is a small add-on job that pays for itself in saved labor over the life of the system.

Septic designer

a.k.a. licensed septic designer, NH-licensed designer

The licensed professional who designs the septic system, applies for the state permit, and signs off on the design. In NH, septic designers are licensed under RSA 485-A and the design plans go to NH DES (Department of Environmental Services) for permit approval. The designer designs and permits; the excavation contractor handles the dig, the tank set, and the install. Two separate roles.

Effluent

The partially-treated wastewater that leaves the septic tank and flows to the leach field for final treatment in the soil. Effluent is not raw sewage; the tank has done some of the work first by separating solids and floating fats. But it's also not clean water, which is why the leach field setbacks from wells, water bodies, and property lines exist.

Soil-dependent septic separation

The minimum distance NH state rule requires between a leach field and a public water body, surface water, or well. The number isn't fixed at one value: NH DES requires separations of about 75 to 125 feet from a water body depending on soil type, slope, and the specific system design. Sandier, faster-perc soils need more horizontal distance because effluent moves faster through them.

Drainage

Curtain drain

a.k.a. interceptor drain

A perforated pipe in a stone-filled trench installed uphill of a structure to intercept groundwater before it reaches the foundation. Common on Lakes Region hillside lots where spring melt and shallow groundwater push water toward foundations and basements. Effective, relatively cheap, and one of the higher-value improvements an older NH home can get if the basement is wet.

French drain

A general term for a perforated pipe in a stone-filled trench used to move water from a wet area to somewhere it can drain. Often used interchangeably with curtain drain, though strictly a curtain drain is uphill of a structure to intercept water, while a French drain can be placed anywhere a wet spot needs draining. The construction is the same: trench, fabric liner, perforated pipe, stone, fabric on top, soil cover.

Foundation drain

a.k.a. perimeter drain, footing drain

The perforated pipe that runs around the outside of a foundation footing to collect water and carry it away from the building. Modern NH foundations get one as standard. Older homes (pre-1970s typically) often don't have one, which is why basement leaks are a common Lakes Region problem on older housing stock. Adding a perimeter drain to an existing foundation is full excavation work (perimeter dig down to the footing), but it solves the problem permanently.

Swale

A shallow, graded depression in the land that moves surface water in a controlled direction. Swales are how driveways shed runoff, how parking lots avoid ponding, and how hillside lots route spring melt around buildings instead of through them. A well-cut swale can replace a more expensive piped drain on many sites. Cheap to build, quiet to maintain.

Materials

Loam (and loam vs. topsoil)

Loam is a soil with a balanced mix of sand, silt, and clay, plus organic matter. It's what you want for lawns, gardens, and grading. Topsoil is a broader term: any dark, organic-rich soil from the top layer of a site. All loam is topsoil, but not all topsoil is loam. Cheap "topsoil" can be high in clay or low in organic content; screened loam from a known supplier is a more consistent product. I deliver screened loam by the yard.

Crushed stone

Mechanically crushed stone, sized by sieve. Common sizes: 3/4-inch (driveway base, drainage stone, septic bedding), 1.5-inch (sub-base, larger drainage), 3/8-inch (top dressing, decorative). The size is named after the largest stone that passes through the sieve. Crushed stone has angular faces (it locks together when compacted), unlike round-faced river stone or pea stone (which doesn't compact and is mostly decorative).

Stone dust

a.k.a. pack, screenings, fines

The fine material that's left after stone is crushed and the larger sizes are sieved out. Stone dust packs into a dense, semi-stable surface (paths, patio bases, driveway top course on some installs). Not a structural material, but useful as a finish layer or fill where you want something denser than gravel that can still be cut and graded.

Bank-run gravel

a.k.a. pit-run gravel, run-of-bank

Unprocessed gravel taken straight from the pit, with whatever mix of sand, gravel, and stones the pit produces. Cheaper than processed gravel, less consistent. Useful for fill, rough base, or jobs where the spec doesn't require a specific grading. Processed gravel (also called "3/4-inch processed" or "crushed gravel") has been sieved to a known size and is what you want for a driveway base.

Mason sand vs. concrete sand

Mason sand (also called fine sand) is a finer grade used for masonry mortar, paver setting beds, and sandbox fill. Concrete sand is coarser, with sharper-edged grains, used for mixing concrete and as septic system bedding. The two are not interchangeable: mason sand in concrete makes weaker concrete; concrete sand in masonry mortar is too gritty to work cleanly.

NH terrain & regulation

Mud season

The stretch in March and early April when the frost is leaving the ground, the snow is melting, and dirt roads turn to soup. NH towns post weight limits on most dirt roads during mud season to keep heavy equipment from rutting them. Most foundation excavation pauses during mud season because subgrades are too saturated to compact properly. Septic and drainage work can sometimes start sooner depending on access and site conditions. A real NH season, not a metaphor.

RSA 483-B (Shoreland Water Quality Protection Act)

The NH state law that controls what can happen within 250 feet of a public water body. The full name is the Shoreland Water Quality Protection Act. Inside the 250-foot zone (the "Protected Shoreland"), excavation, tree removal, impervious surface, and structures are all regulated. The first 50 feet from the reference line (the "Waterfront Buffer") has the strictest rules. Most Lakes Region waterfront work touches RSA 483-B in some way, and a Shoreland Permit may be required before the dig depending on scope. Read the full statute →

Reference line (waterbody)

The line from which the 250-foot Protected Shoreland and the 50-foot Waterfront Buffer are measured under RSA 483-B. For a lake or pond, the reference line is the natural mean high-water mark or the maintained waterline of an artificially impounded waterbody. For a river, it's the ordinary high-water mark. The reference line matters because every Shoreland setback and permit requirement is measured from it.

Missing a term you ran into on a quote or a permit? Text Shawn (603) 832-8315 and I'll add it.